If soft-bodied arthropods such as mites, scale, aphids, thrips, mealybugs, or whiteflies are causing significant damage in your garden, one possible chemical method for controlling them is horticultural oil. Horticultural oil is often made from highly refined petroleum oils to which an emulsifying agent has been added so that the oil will mix with water. Sometimes vegetable oils such as cottonseed oil, soybean oil or neem oil are used instead of the mineral oils. Some types are OMRI approved for organic use. Some have also been used successfully against certain fungi such as powdery mildew. These oils are easily diluted and sprayed on the affected plant using any hose end or tank style sprayer.
In the past, oils such as these were called “dormant oil” and, because of their potential to cause harm to foliage, they could only be used during dormant seasons to kill overwintering insects, mites and eggs. However, improved refinement techniques have purified them even more by removing most of the sulfur-rich impurities that could damage plants during the growing season. For that reason the newer lighter oils are sometimes called “summer oils” or “superior oils”. Today, summer oil and dormant oil really only refer to the time and rate of application. The oil you purchase can be used for either application. Generally, a 1 to 2 % solution is used during the growing season and a 3 to 4 % solution during the dormant season, but be sure to carefully follow the instructions on the label of each brand!
Horticultural oil controls pests in several ways. It can clog the breathing tubes of the insect or mite causing suffocation, or it may act as a poison affecting their cell membranes. In some cases, the oil disturbs the feeding behavior of the pest. The benefit of using it is that it is less toxic than synthetic insecticides for people and other animals. It evaporates fairly quickly and has no residual activity so that it is less harmful to beneficial insects, and the pests that it does affect do not become resistant to it. However, it does still have the potential to harm certain plants, and some plants are more sensitive to it than others. These include certain maples, walnuts, hickories, smoke trees, and cryptomeria, and to a lesser extent some junipers, cedars, spruces, and douglas fir. It can change the color of blue-tinged conifers. This is not a complete list, so it is important when using it to first read the label to see what plants might be affected by that formulation! If you are concerned that it may harm the plant you want to use it on, try spraying a small section of the plant first.
There are certain rules you should follow if you want to use this oil. Use only between the temperatures of 40 and 90 degrees Fahrenheit, but check the instructions to make sure temperatures aren’t even more restricted than this. Don’t apply if the foliage is wet or humidity high (greater than 90%)since the longer wet oil stays on the plant, the greater the chance for plant damage. Don’t use oil during drought conditions since plants are more prone to damage at this time as well. Watering a day or two ahead of time is a good idea to make sure the plants aren’t drought stressed. Don’t combine the oil with sulfur containing insecticides. Don’t spray when new shoots are rapidly growing in the spring or in the fall after September in the Northeast. When applying it with a sprayer, make sure to agitate frequently so it remains well mixed. Used properly, horticultural oil can be a safe and effective method to control some of the most common garden pests!